FLINT, MI -- Approximately 10 percent of homes tested by a Wayne State University-led team in Flint had relatively low levels of chlorine in tap water, according to updated information released by researchers.

The Flint Area Community Health and Environmental Partnership team, formed to study Legionnaires' disease in Flint for the state Department of Health and Human Services, found chlorine levels of less than 0.2 milligrams per liter at one in every 10 homes tested from Sept. 6 until Oct. 29 in 2016 and from May through September this year.

"It's an issue that needs further investigation," said Wayne State spokesman Matt Lockwood.

Chlorine is added to water distribution systems to reduce the growth of harmful bacteria, including Legionella, and low levels of chlorine in parts of Flint's water system have been a concern of the U.S. Environmental Protection Agency since before the agency issued an emergency order in January 2016.

A news release issued by Wayne State says a uniform standard does not exist for residual chlorine but regulatory agencies typically recommend maintaining a free chlorine residual of 0.2 to 0.5 mg/L.

A lack of residual chlorine is a special concern in Flint, officials have said, because pockets of the water system have had problems with stagnation due to low water use as the city's population has steadily declined.

Marc Edwards, a professor at Virginia Tech University, said considering the age and composition of the Flint water system, the chlorine results are good.

"On the basis of my experience, in systems with old unlined iron pipes, having 10 percent or less of homes with chlorine less than 0.2 mg/L is quite good," Edwards said in an email to MLive-The Flint Journal. "On a relative basis, both VT and EPA have tracked levels of chlorine pre, during and post-crisis. At the same sampling sites and using the same sampling methods, chlorine levels are higher now than pre-crisis, and much higher than during the crisis."

Although an analysis of tests in Flint, Genesee and Wayne counties for Legionella from 2017 is still in progress, an analysis of samples from homes sampled in 2016, found culturable Legionella in approximately 12 percent of homes sampled.

Genesee County suffered a surge in Legionnaires' disease in parts of 2014 and 2015 after the city changed its water source from pre-treated water from Lake Huron to raw water from the Flint River in April 2014.

At least a dozen people died as a result of the outbreak, prompting the sampling by the FACHEP group.

Edwards said finding 12 percent of homes with culturable Legionella is not unusual and the Legionella disease incidence in Flint is returning to historical norms.

In Flint, water from more than 370 homes have been sampled since the start of the program while 272 home in Genesee County and 147 homes in Wayne County were also sampled to provide comparison information to the Flint data, Wayne State's news release says.

The chlorine residual testing measured disinfection levels at the kitchen faucet of homes, bypassing any filters installed, after 5 minutes of flushing.